Extrusion dies are useful in forming cellular or honeycomb ceramic substrates for use in catalytic converters, which are utilized in exhaust systems of internal combustion engines. In order to reduce back pressure within the exhaust system, it is necessary that the cell walls or webs have a substantially thin cross-sectional dimension so as to provide a substantially large open frontal area. However, the thin walled structure must be protected so as to withstand normal automotive impact requirements. One way to protect the inner cells is to increase the thickness of the cell walls in peripheral portions of the extruded ceramic substrate, thus providing additional strength to withstand external loads.
European Patent Specification EP 0 674 018 B1 teaches the use of blocking plates during the recoating of dies to obtain uniform slot width. The blocking plates are used to limit the amount of coating applied to worn dies that need to be recoated and be within constant web thickness. U.S. Pat. No. 5,952,079 uses a mask in the form of a Teflon tape to block off outer portions of a die during liquid nickel plating, so as to inhibit the occurrence of edge chipping of a honeycomb structure. Problems can arise in connection with the latter method, however, in that it is difficult to control plating gradients and to achieve uniform results under varying conditions of liquid flow and tape alignment.
The present invention overcomes problems encountered in the prior art by setting forth a controlled approach for obtaining desired web thicknesses of a honeycomb structure, by controlling the flow of gases within slots of a honeycomb extrusion die during the chemical vapor deposition (CVD) coating of the die.